The invention relates to time domain reflectometer (TDR) apparatus and particularly to apparatus which enable the user to identify the location of defects such as breaks in a cable very close to the point where the time domain reflectometer apparatus is attached to the cable.
A time domain reflectometer is an instrument designed to indicate and to measure reflection characteristics of a transmission system connected to the instrument by monitoring the step-formed signals entering the test object and the superimposed reflected transient signals on an oscilloscope equipped with a suitable time-base sweep. The measuring system, basically, consists of a fast-rise step-function generator, a tee coupler, and an oscilloscope connected to the probing branch of the coupler.
When the transmission line, "under test", includes passive devices which are not capable of returning the low frequency components of the wave form, then a short duration pulse wave form is sometimes substitued for the step function wave form.
A disadvantage of such apparatus which is generally available is that it is difficult to distinguish between a break in the center conductor which is very close to the point which is attached to the output port of the reflectometer and a perfect cable which is terminated. The difficulty arises because the reflected pulse is masked by the "zero pulse" or step-formed signals which are being introduced into the cable. Stated another way, the physical display of the "zero pulse" and the reflected pulse will be substantially superimposed and thus the display will appear as a single pulse, which would indicate a perfect cable. This is obviously undesirable since the fault is not detected.
Time domain reflectometers are typically used to test radio frequency transmission cables such as 50 or 75 ohm cables. The invention has particular application to testing of coaxial cables such as those used for cable television. Other applications include telephone cables and other lines which have a somewhat uniform impedance. The circuit values given herein will be understood to refer to 75 ohm cable testing although those skilled in the art will understand how the concepts disclosed herein may be utilized in testing other lines. Television and telephone cables may often be buried underground and may be broken accidentally, such as during installation or repair of other services. The identification of the spot in the cable in which the break has occurred may be a difficult procedure.
Another problem arises when a low frequency signal is present on the cable to be tested. For example, it is quite common to have a 60 V ac 60 Hz signal on the cable of a Cable TV system. This ac signal will make it impossible to identify the TDR trace.
It is an object of the invention to provide apparatus which enables the simple and inexpensive testing of a cable which will identify defects which are disposed even at a point which is close to the point at which the time domain reflectometer apparatus is attached to the cable being tested.
Another object of the invention is to provide apparatus which will greatly reduce the display of the 60 Hz signal which may be present on the cable such that the TDR display will not be affected.